1. Field of the Invention
The present embodiments described herein relates generally to RFID systems, and more particularly to systems and methods for RF nulling within an RFID system.
2. Background of the Invention
FIG. 1 is a diagram illustrating an exemplary RFID system 100. In system 100, RFID interrogator 102 communicates with one or more RFID tags 110. Data can be exchanged between interrogator 102 and RFID tag 110 via radio transmit signal 108 and radio receive signal 112. RFID interrogator 102 comprises RF transceiver 104, which contains transmitter and receiver electronics, and antenna 106, which are configured to generate and receive radio transmit signal 108 and radio receive signal 112, respectively. Exchange of data can be accomplished via electromagnetic or electrostatic coupling in the RF spectrum in combination with various modulation and encoding schemes. RFID tag 110 is a transponder that can be attached to an object of interest and act as an information storage mechanism. In many applications, the use of passive RFID tags is desirable, because they have a virtually unlimited operational lifetime and can be smaller, lighter, and cheaper than active RFID tags that contain an internal power source, e.g. a battery. Passive RFID tags power themselves by rectifying the RF signal emitted by the RF scanner. Consequently, the range of transmit signal 108 determines the operational range of RFID tag 110.
RF transceiver 104 transmits RF signals 108 to RFID tag 110, and receives RF signals 112 from RFID tag 110, via antenna 106. The data in transmit signal 108 and receive signal 112 can be contained in one or more bits for the purpose of providing identification and other information relevant to the particular RFID tag application. When RFID tag 110 passes within the range of the radio frequency magnetic field emitted by antenna 106, RFID tag 110 is excited and transmits data back to RF interrogator 102. A change in the impedance of RFID tag 110 can be used to signal the data to RF interrogator 102 via receive signal 112. The impedance change in RFID tag 110 can be caused by producing a short circuit across the tag's antenna connections (not shown) in bursts of very short duration. RF transceiver 104 senses the impedance change as a change in the level of reflected or backscattered energy arriving at antenna 106.
Digital electronics 114, which can comprise a microprocessor with RAM, performs decoding and reading of receive signal 112. Similarly, digital electronics 114 performs the coding of transmit signal 108. Thus, RF interrogator 102 facilitates the reading or writing of data to RFID tags 110, that are within range of the RF field emitted by antenna 106. Together, RF transceiver 104 and digital electronics 114 comprise interrogator 102. Finally, digital electronics 114 can be interfaced with an integral display and/or provide a parallel or serial communications interface to a host computer or industrial controller, e.g. host computer 116.
Generally, RFID systems 100 must receive a backscatter signal 112 from tag 110 while transmitting signal 108. Simultaneous transmission and reception can cause high levels of RF energy to enter the receiver, ultimately limiting the receiver sensitivity. Existing system designs attempt to solve this problem by either minimizing the signal reflections back into the receiver or by using separate transmit and receive antennas. Minimizing signal reflections via component selection has practical limitations. For example, it can be difficult to perfectly match impedance because of variability in the manufactured components used in a device or system. These impedance mismatches can cause reflections. Further, using separate antennas increases the system cost and requires additional space.